1. Field of the Invention
The present invention relates to a machining method for a wiring board and a machining apparatus for a wiring board employing a laser beam for machining such as drilling for a through-hole, an inner via hole, and a blind via hole, grooving, and cutting for an outside shape of the wiring board referred to as so-called printed board, and more particularly to a machining method for a wiring board and a machining apparatus for a wiring board in which a fine conduction hole can rapidly and accurately be formed, and a carbonic acid gas laser oscillator to generate a pulsed laser beam most suitable for the above machining.
2. Description of the Prior Art
A printed board includes a plurality of insulating base materials with conductor layers, stacked and joined in a multi-layer fashion. Among the conductor layers applied onto the insulating base materials, the optional conductor layers in a vertical direction are electrically connected through conduction holes referred to as a through-hole, an inner via hole, and a blind via hole. FIG. 33 is a sectional view of such a conventional multi-layer printed board. In the drawing, reference numeral 51 means a printed board, 52 to 56 are conductor layers, 57 is metallic deposits, 61 to 64 are insulating base materials, and 65 to 68 are conduction holes. In the five-layer printed board 51 including the conductor layers 52 to 56, the insulating base materials 61 and 63 with both sides coated with copper foil and the conductor layer 56 including copper foil are stacked and joined by using the insulating base materials 62 and 64 referred to as prepreg, and the conduction holes 65 to 68 are provided to pass through the conductor layers 52 to 56.
As shown in FIG. 33, the conduction hole 65 is mounted for conduction between the conductor layer 52 and the conductor layer 53 in the insulating base material 61, and the conduction hole 66, referred to as blind via hole (BVH), is mounted for conduction between the conductor layer 52 in the insulating base material 61 and the conductor layer 54 in the insulating base material 63. The conduction hole 67, referred to as inner via hole (IVH), is mounted for conduction between the conductor layer 54 and the conductor layer 55 in the insulating base material 63. The conduction hole 68, referred to as through-hole (TH), is mounted for conduction between the conductor layer 52 in the insulating base material 61 and the conductor layer 56 stacked and joined through the insulating base material 64.
The conduction holes 65 to 68 shown in FIG. 33 are holes machined by a drill. Further, after drilling, the conduction holes are plated through the metallic deposits 57, and the conductor layers are electrically connected.
In the prior art, a machining method for the conduction hole includes, for example, drill machining using a rotary milling cutter. Further, a machining method for grooving or cutting for an outside shape includes, for example, router machining using a rotary milling cutter. On the other hand, in recent years, higher density wiring has been desired for higher performance of an electronic device. A more multi-layered and smaller printed board is required to meet the above requirement. Further, it is essential to provide a finer hole diameter of the conduction hole for this purpose. With the current state of the art, the conduction hole is generally provided in the printed board by the mechanical method using the drill. However, the method has drawbacks in that the finer hole diameter is limited, for example because drilling a hole diameter of xcfx860.2 mm or less is very difficult to cause heavy wear of the drill such as breakage, resulting in poor productivity due to the long time required for replacement of the drill. Further, it is difficult to simultaneously machine adjacent positions, thereby requiring a considerable machining time. In addition, the insulating base material has a thickness of 0.1 mm or less because of the smaller printed board. Since it is difficult to control a hole depth in the drill machining with accuracy of 0.1 mm or less, it is difficult to form a blind via hole in such a thin-walled insulating base material. Further, in order to realize cost reduction by the smaller printed board and an increase in yield, the grooving and the cutting for the outside shape require an accurate depth control in the grooving, a narrower cutting width, and cutting after parts are packaged. However, the mechanical methods such as router machining are unpractical since the above limitation is similarly imposed thereon.
Instead of the machining methods for the printed board including the above mechanical methods, attention has been given to methods, which have partially been put to practical use, employing a laser beam such as an eximer laser or carbonic acid gas laser, disclosed in IBM Journal of Research and Development, Vol. 126, No. 3, pp. 306-317 (1982), and Japanese Patent Publication (Kokoku) No. 4-3676. These laser beam machining methods utilize a difference in absorption coefficient of light energy such as the eximer laser or the carbonic acid gas laser between resin or glass fiber serving as the insulating base material forming the printed board, and copper serving as the conductor layer. For example, since almost the entire laser beam emitted from the above laser can be reflected at the copper, a copper foil removed portion having a required diameter is formed in top copper foil through etching and so forth, and the copper foil removed portion may be irradiated with the laser beam. It is thereby possible to selectively decompose and remove the resin and the glass so as to form a fine through-hole and a fine inner via hole in a short time. If internal-layer copper foil is previously stacked in a machined portion, the decomposition and the removal of the insulating base material are terminated at the internal-layer copper foil. It is thereby possible to form a blind via hole which can surely be terminated at bottom copper foil. There is an advantage of no wear of the tool such as breakage because the laser beam machining methods are contactless machining methods.
The above laser beam machining methods employ a pulse laser such as the eximer laser and a TEA-carbonic acid gas laser, with an extremely narrow pulse width of 1 xcexcs or less. The pulse laser can finely divide into chips (1) a single base material made of high polymeric material such as polyimide, or epoxy, (2) a composite material reinforced by aramid fiber or the like, containing the polyimide, the epoxy, and so forth, and (3) an inorganic material such as glass. It is thereby possible to rapidly and accurately form a good machined hole with a smooth machined portion and less altered layer in a printed board in which as the insulating base material is used composite material dispersed in the polyimide, the epoxy, and so forth.
The conventional laser beam machining method for the wiring board has the above structure. The eximer laser or the TEA-carbonic acid gas laser is used to provide the through-hole and the inner via hole in the most commonly used printed board having the insulating base material made of glass cloth containing the glass fiber and the resin, such as a glass epoxy printed board referred to as FR-4 made of the glass cloth and epoxy resin. In this case, there are problems in that metallic deposit for conduction can not easily be coated on a hole inner wall due to the extremely rough inner wall of the hole, and reliability of the metallic deposit can not be ensured. The problems are generated because the insulating material of the printed board is not only the composite material made of organic material and inorganic material but also heterogeneous material in which the organic material and the inorganic material are contained in clusters to some extent.
Further, there is another problem in that a uniform machined hole can not be provided due to differences in, for example, absorption coefficient of the laser beam, decomposition temperature, and thermal diffusivity between an organic material portion and an inorganic material portion. For example, since a wavelength of the laser beam in the eximer laser can not easily be absorbed by the glass, sufficient energy for decomposition of the glass can not be supplied so that a glass portion is difficult to remove, resulting in a problem of a rough machined hole. On the other hand, both the resin and glass can show high absorption coefficient in case of the TEA-carbonic acid gas laser. However, when energy density of 20 J/cm2 required to efficiently machine glass epoxy material is obtained in the TEA-carbonic acid gas laser, excessively high power density of 2xc3x97107 W/cm2 or more is caused due to the narrow pulse width of 1 xcexcs or less. Such high power density may easily cause plasma at the machined portion. Once the plasma is formed, laser energy is absorbed by the plasma, resulting in insufficient energy reaching the machined portion. Hence, it is difficult to remove glass having a high decomposition temperature, thereby causing a problem of the rough machined hole.
If the energy density is set to cause no plasma, the machining progresses extremely slowly, resulting in a problem of a reduction in productivity.
In addition, only when a beam diameter is larger than the machined portion, good machining may be made to the materials (1), (2), and (3) in the conventional laser beam machining method. Otherwise, if the machined portion is larger than the beam diameter, for example, in case of cutting, grooving, and drilling for a large diameter hole, a removed material caused at a beam irradiated portion adheres to a position other than the beam irradiated portion. As a result, after the machining, the machined portion is coated with additionally deposited soot, thereby reducing reliability of insulation and reliability of metallic deposit in the printed board. Hence, there is another problem of the need for the step of, for example, complicated after-treatment such as wet etching.
Other than the extremely short pulse laser such as the eximer laser and the TEA-carbonic acid gas laser, there are other laser beam machining methods for the wiring board, using a typical carbonic acid gas laser of high-speed axial-flow type or three-axes orthogonal type in the prior art. In the conventional carbonic acid gas lasers, more importance is given to a CW output characteristic than a pulse output characteristic to enhance oscillation efficiency. That is, there is in theory a problem of pulse response sensitivity at a time of pulse oscillation, in particular, a characteristic in which a time is required for a fall of a laser pulse. Thus, the machining by the conventional carbonic acid gas laser having such a characteristic results in irradiation of the machined portion with a laser beam for a longer time, thereby causing a gradual temperature gradient around the machined portion. As a result, as shown in FIG. 34, a larger difference is caused in amount of removal between the resin and the glass due to a difference in decomposition temperature therebetween. When only the resin is excessively removed, there are problems in that projection of the glass fibers results in a rough machined hole as shown in FIG. 35, and the long heating time generates a char layer on a wall surface of the hole.
Further, carbides are generated around the machined portion, and the laser beam is absorbed by the copper through the carbides, resulting in frequent damage to the copper foil as shown in FIG. 36. Hence, there is a problem in that the blind via hole is difficult to form in the above laser beam machining methods.
Though descriptions have been given of the machining for the hole, the same problems are caused in the grooving and the cutting.
In order to overcome the above problems, it is an object of the present invention to provide a stable laser beam machining method for a wiring board, in which a printed board with an insulating base material containing cloth-like glass fibers can rapidly and accurately be machined, for example, drilled for a through-hole, an inner via hole and a blind via hole, grooved, or cut for an outside shape without roughness of a machined portion and the need for complicated after-treatment of additional deposit, and no damage is caused to copper foil, and to provide a laser beam machining apparatus for a wiring board, for realizing the laser beam machining method for the wiring board and improving productivity.
It is another object of the present invention to provide a carbonic acid gas laser oscillator for machining a wiring board, which can output a laser beam with a pulse width most suitable for the laser beam machining method for the wiring board.
According to one aspect of the present invention, for achieving the above-mentioned objects, there is provided a laser beam machining method for a wiring board, including the step of irradiating a machined portion of the wiring board with a pulsed laser beam for a beam irradiation time ranging from 10 to 200 xcexcs and with energy density of 20 J/cm2 or more.
According to another aspect of the present invention, there is provided a laser beam machining method for a wiring board, including the step of irradiating the same machined portion of the wiring board with a pulsed laser beam with intervals of a beam irradiation pausing time of 15 ms or more and energy density of 20 J/cm2 or more.
According to still another aspect of the present invention, there is provided a laser beam machining method for a wiring board, including the steps of combining into one pulse group laser beams including a plurality of pulses having energy density of 20 J/cm2 or more and generated at intervals of a predetermined beam irradiation pausing time, and irradiating the same machined portion of the wiring board with a pulsed laser beam with the plurality of pulse groups respectively including the plurality of pulses at intervals of a pulse group interval irradiation pausing time longer than the predetermined beam irradiation pausing time. Preferably, the predetermined beam irradiation pausing time is 4 ms or more, the number of pulses in the pulse group is 4 or less, and the pulse group interval irradiation pausing time exceeds 20 ms.
According to a further aspect of the present invention, there is provided a laser beam machining method for a wiring board, including the step of, at a time of scanning a surface of the wiring board while irradiating a machined portion of the wiring board with the pulsed laser beam, scanning by a laser beam such that the machined portion is not continuously irradiated with the laser beam over 4 pulses at intervals of a beam irradiation pausing time less than 15 ms.
According to a still further aspect of the present invention, there is provided a laser beam machining method for a wiring board, including the steps of providing a 1 mm beam diameter on a surface of a machined portion, and scanning a surface of the wiring board at a scanning speed ranging from 8 to 6 m/min while irradiating the machined portion with a laser beam for a beam irradiation time ranging from 10 to 200 xcexcs and at intervals of a beam irradiation pausing time of 2.5 ms.
According to another aspect of the present invention, there is provided a laser beam machining method for a wiring board, including the steps of setting a laser beam to have a square spot effective in machining of a machined portion of the wiring board, and scanning a surface of the wiring board while irradiating the machined portion of the wiring board with the pulsed laser beam. Preferably, the square spot of the laser beam on the machined portion is set to have a size of 0.9 mmxc3x970.9 mm, and the surface of the wiring board is scanned with a scanning speed of 6 m/min and a scanning pitch of 200 xcexcm while the machined portion is irradiated with the laser beam for a beam irradiation time ranging from 10 to 200 xcexcs and at intervals of a beam irradiation pausing time of 1.25 ms.
According to a further aspect of the present invention, there is provided a laser beam machining method for a wiring board, including the steps of previously removing a metallic layer on the wiring board at a portion corresponding to a machined portion of the wiring board, forming a base material removed portion through machining by irradiating a base material of the machined portion with a laser beam through the metallic layer removed portion, and additionally irradiating the base material removed portion and a periphery of the base material removed portion, or only the periphery of the base material removed portion with a laser beam. Preferably, the additionally irradiated laser beam has a smaller peak output than a peak output of the first laser beam, and is used to scan at a higher scanning speed than a scanning speed during first laser beam irradiation.
According to a further aspect of the present invention, there is provided a laser beam machining method for a wiring board, including the step of, at a time of previously removing a metallic layer on the wiring board at a portion corresponding to a machined portion, partially removing the metallic layer such that a laser beam can reach only an outer periphery of a base material removed portion to be formed by irradiating a base material of the machined portion with the laser beam. Preferably, a surface of the wiring board is scanned with a scanning speed of 8 m/min and a scanning pitch of 100 xcexcm while the machined portion being irradiated with the laser beam for a beam irradiation time ranging from 10 to 200 xcexcs and at intervals of a beam irradiation pausing time of 2.5 ms.
According to a further aspect of the present invention, there is provided a laser beam machining method for a wiring board, including the steps of previously removing a metallic layer on the wiring board at a portion corresponding to a machined portion of the wiring board, and flowing a gas in a direction from a laser beam scanning start point to a laser beam scanning end point in the machined portion at a time of machining by irradiating a base material of the machined portion with a laser beam while scanning by the laser beam through the metallic layer removed portion.
According to a further aspect of the present invention, there is provided a laser beam machining method for a wiring board, including the steps of forming a metallic layer having a desired shape by partially removing the metallic layer by pulse irradiation with a laser beam having sufficient intensity to melt and remove the metallic layer on the wiring board, and additionally irradiating a machined portion of the wiring board through a metallic layer removed portion with the laser beam having insufficient intensity to melt the metallic layer and a beam irradiation time ranging from 10 to 200 xcexcs, and including a plurality of pulses forming a train at intervals of a beam irradiation pausing time of 15 ms or more. Preferably, the machined portion is exposed by previously removing, through another machining method such as etching, the metallic layer positioned at a target position for laser beam irradiation and in the range smaller than a shape to be machined. Further, surface roughening may previously be made to a surface of the metallic layer on a surface of the wiring board before the laser beam irradiation.
According to a further aspect of the present invention, there is provided a laser beam machining method for a wiring board, including the step of, at a time of pulse irradiation with a laser beam while sequentially positioning a spot of the laser beam at target positions on the wiring board in synchronization with a pulse frequency of the laser beam, providing a time interval of 15 ms or more between two optional successive pulsed laser beams for irradiation of the respective target positions irrespective of the pulse frequency by irradiating another target position with a pulsed laser beam outputted for the time interval therebetween.
According to a further aspect of the present invention, there is provided a laser beam machining method for a wiring board, including the steps of providing a plurality of machining stations on which the wiring boards to be machined are mounted, sequentially dividing a pulsed laser beam outputted from a laser oscillator among the plurality of machining stations for each pulse, and introducing the pulsed laser beam into the plurality of machining stations at time intervals of 15 ms or more. Preferably, a carbonic acid gas laser is used as a light source of the laser beam. The wiring board may contain glass cloth.
According to a further aspect of the present invention, there is provided a laser beam machining apparatus for a wiring board, including an optical mechanism to change a direction of a laser beam and move the laser beam on the wiring board while sequentially positioning a spot of the laser beam at target positions on the wiring board, and a control mechanism for synchronous control between a pulse oscillating operation of a laser oscillator and an operation of the optical mechanism, and control of the optical mechanism such that a time interval can be set to 15 ms or more between two optional successive pulsed laser beams for irradiation of the target positions irrespective of a pulse frequency of the laser oscillator.
According to a further aspect of the present invention, there is provided a laser beam machining apparatus for a wiring board, including an optical mechanism to sequentially divide a pulsed laser beam outputted from a laser oscillator among a plurality of machining stations for each pulse and introduce the pulsed laser beam into the plurality of machining stations for each pulse at time intervals of 15 ms or more, and a synchronization control mechanism for synchronous control between a dividing operation of the optical mechanism and a pulse oscillating operation of the laser oscillator. Preferably, the optical mechanism is provided with at least one rotary chopper rotated at a predetermined speed of rotation, having a plurality of reflection surfaces and a plurality of passing portions at positions equally dividing a periphery about an axis in a plane perpendicular to the rotation axis. Further, the synchronization control mechanism is provided with a trigger generating apparatus to generate a trigger each time all the equally divided areas including the plurality of reflection surfaces and the plurality of passing portions in the rotary chopper respectively move across an optical axis of the laser beam.
According to a further aspect of the present invention, there is provided a carbonic acid gas laser oscillator for machining a wiring board, in which a length of a discharge space in a gas flow direction is equal to or more than a width of an aperture, an optical axis passing through a center of the aperture is set to be positioned in the range that an entire area of the aperture does not extend off an area extending in the gas flow direction of the discharge space and on the farthest upstream side of the gas flow, and a rise time and a fall time are set to 50 xcexcs or less in discharge power fed to the discharge space.
The above and further objects and novel features of the invention will more fully appear from the following detailed description when the same is read in connection with the accompanying drawings. It is to be expressly understood, however, that the drawings are for purpose of illustration only and are not intended as a definition of the limits of the invention.